Bartonella (Rochalimaea) species have recently become associated with four clinical syndromes: Bacillary angiomatosis (BA); Bacillary peliosis hepatitis (BHP); Relapsing fever with bacteremia; Cat-scratch disease (CSD). Individuals at risk for BA, BHP, bacteremia, and CSD infections include adults with human immunodeficiency virus, transplant patients, and other immuno-compromised patients. Immunocompetent adults and children may also develop CSD. In primary or relapsed trench fever, only antibodies to Bartonella quintana are present. Bartonella quintana antibodies may be cross-reactive with Rickettsia vinsonii, and cross-reactions are obtained with approximately 50 percent of typhus group (Rickettsia prowazekii and Rickettsia mooseri), as well as 66 percent of scrub typhus (Rickettsia tsutsugamushi) sera. Bartonella quintana sera, however, are not commonly cross-reactive with sera from spotted fever group organisms (Rickettsia rickettsia and Rickettsia akari). Bartonella quintana antibodies may be seen in patients with bacillary angiomatosis and/or parenchymal bacillary peliosis. A low positive suggests past exposure or infection, while high positive results may indicate recent or current infection, but are inconclusive for diagnosis. Seroconversion between acute and convalescent sera is considered strong evidence of recent infection. The best evidence for infection is a significant change on two appropriately timed specimens where both tests are done in the same laboratory at the same time. While the presence of IgM antibodies suggest recent infection, low levels of IgM antibodies may occasionally persist for more than 12 months post-infection.